1. Field of the Invention
The present invention relates generally to an eddy current measuring system, and in particular to an eddy current measuring system for estimating the thickness of conductive films formed on semiconductor wafer products.
2. Description of the Related Art
In the semiconductor industry, critical steps in the production of semiconductor wafers are the selective formation and removal of films on an underlying substrate. The films are made from a variety of substances, and can be conductive (for example, metal or a magnetic ferrous conductive material) or non-conductive (for example, an insulator or a magnetic ferrite insulating material).
Films are used in typical semiconductor processing by: (1) depositing a film; (2) patterning areas of the film using lithography and etching; (3) depositing material which fills the etched areas; and (4) planarizing the structure by etching or chemical-mechanical polishing (CMP). Films may be formed on a substrate by a variety of well-known methods including physical vapor deposition (PVD) by sputtering or evaporation, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and electro-chemical process (ECP). Films may be removed by any of several well-known methods including chemical-mechanical polishing (CMP), reactive ion etching (RIE), wet etching, electrochemical etching, vapor etching, and spray etching.
The semiconductor fabrication industry continues to demand higher yields and shorter fabrication times, while insisting upon ever-increasing quality standards. A variety of inspection procedures have been employed during the various stages of the semiconductor wafer fabrication process in an attempt to meet these demands. These inspection procedures include destructive, as well as nondestructive, testing methods for analyzing wafer products. In a destructive measuring process, a standard or electron microscope may be used to measure the thickness of a wafer's coating after a cross-section has been obtained. When the thickness of a thin-film coating is greater than 10,000 Å, for example, this type of destructive measuring method may provide accurate measurements. However, measuring accuracy usually begins to degrade as the coating thickness falls below the 10,000 Å threshold.
Other types of measuring processes utilize sensitive eddy current sensors which do not destroy or significantly alter the article measured. Although eddy current sensors provide highly accurate readings, these sensors are susceptible to error. For example, the shifting of an electronic reference point due to thermal drifting often occurs at some point during the data collection and inspection process. To compensate for thermal drifting and to ensure accurate readings, many existing eddy current sensors must be recalibrated on a periodic basis.
While there have been other attempts in addition to eddy current sensors to employ highly accurate, nondestructive measuring devices for estimating the thickness of a conductive top layer formed on a semiconductor wafer product, improvement is still needed.